Cypress Upload custom command.

CypressUpload.js

Cypress.Commands.add('upload_file', (fileName, selector) => {
     return cy.get(selector).then(subject => {
         return cy.fixture(fileName, 'base64')
         .then(Cypress.Blob.base64StringToBlob)
         .then(blob => {
             const el = subject[0]
             const testFile = new File([blob], fileName, { type: 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet' })
             const dataTransfer = new DataTransfer()
             dataTransfer.items.add(testFile)
             el.files = dataTransfer.files
             return subject;
         })
     })
 })

Example

cy.upload_file(file or 'string.extension', 'selector')

Selector can be css class/id or by input.